1. Field of the Invention
The present invention relates to the field of power conditioning equipment and, particularly, to control apparatus therefor.
2. Description of Prior Art
A non-dissipatively regulated power conditioning apparatus achieves voltage transformation and regulation through cyclic operation of its power switch. Consequently, a control system must be able to convert analog signals derived from the equipment output and from a control reference, through an analog-to-digital process, into discrete time intervals representing the ON and OFF states of the power switch. The modulated pulse train derived from the cyclic operation of the power switch is reconverted to analog form at the equipment output, by means of an energy storage element, ordinarily a low pass filter.
In conventional non-dissipative power conditioning equipment, this analog-to-digital-to-analog process is accomplished as shown in FIG. 1. Here the main power train is shown to comprise a series chopper, although it may also consist of a boost regulator, buck boost regulator, parallel inverter, series resonant inverters, or other conventional switching means. As shown, the series chopper comprises a switch 10, a "free-wheeling" diode CR, an energy storage element 20 (typically an LC low-pass filter) and the load resistor R.sub.L. The switch itself can consist of any conventional switching device, such as a transistor or silicon-controlled rectifier (SCR).
The equipment DC output signal E.sub.o is processed by an analog control signal processor (ACSP) 25. The ACSP includes a summing junction 30 at which the output DC signal is compared with a reference voltage E.sub.R to generate a DC error signal. The latter is fed to a properly compensated amplifier 35 resulting in a signal to a second summing junction 40.
Here the amplified output is either combined with or transformed into a ramp function. The ramp function can be a flux ramp, via a magnetic device, or it can be a voltage ramp obtained, for example, by charging a capacitor. In any event, when the instantaneous ramp voltage output reaches a threshold level, it causes a threshold detector 45 (typically a Schmitt trigger) to change state.
This causes a pulse output to be passed from the threshold detector to a digital control signal processor (DCSP) 50. The DCSP in its most general sense, comprises a memory and timer, the timer being reset upon receipt of an input from the threshold detector. The output from the DCSP provides an input to the switch 10.
The DCSP 50 may be mechanized in various ways to provide any of the following combinations of on-time T.sub.N and off-time T.sub.F of the power switch 10: (a) constant T.sub.N, variable T.sub.F (b) constant T.sub.F, variable T.sub.N (c) constant (T.sub.N + T.sub.F), variable T.sub.N and T.sub.F and (d) variable (T.sub.N + T.sub.F), variable T.sub.N and T.sub.F.
The most straight-forward embodiment of the DCSP 50, which provides a constant T.sub.N and a variable T.sub.F (determined by the feedback loop), is a simple one-shot multivibrator. In this embodiment the output pulse from the threshold detector 45 activates the multivibrator which generates a pulse of specified duration (the on-time of the power switch) by which the power switch is operated.
The basic circuit shown in FIG. 1 is typically implemented in various embodiments, the variations primarily due to different means by which the ramp function and the threshold level E.sub.T, of the threshold detector, are mechanized. However, this apparatus, which is based on the use of a single-loop control, suffers from certain inherent limitations. The presence of the second-order energy storage element 20 (typically an LC filter) for output ripple reduction, and the necessary use of a high gain, wide bandwidth ACSP amplifier 35 for good static regulation, high audio susceptibility reduction and fast dynamic response usually results in the risk of dynamic instability. The latter can be induced by line or load changes and variation in electrical component characteristics. Another limitation of this method is the fact that the usual long time constant associated with the energy storage element delays the rate of power switch modulation adjustment in response to dynamic line and/or load disturbances, thus compromising the dynamic response of the apparatus.
These inherent limitations are partially alleviated where the ramp of the threshold level, E.sub.T, is generated as a function of the equipment input voltage E.sub.i, rather than as an independent input. Here static and dynamic regulation against line changes may be improved in an open loop fashion by utilization of a line dependent ramp or threshold level. However, the dynamic performance against load changes is again hampered by the output energy storage element 20.
Other embodiments of such a single-loop feedback system exist. However all present one or more inherent limitations.